The present invention relates to a semiconductor device, in particular to a semiconductor device which is capable of preventing solder junction planes of solder balls from being removed therefrom even though mechanical stress is applied to reinforcing pads provided on the four corners of CSP (Chip Size Package).
Conventionally, the CSP, which is efficiently mounted as an LSI (large scale integration) circuit, is mounted on a printed wiring substrate by soldering in a handy equipment, such as a personal telephone, and the like. In such a case, when the handy equipment using the CSP is dropped or an input key of the handy equipment using the CSP is pushed on, mechanical stress is inevitably caused in solder junction portions of the CSP due to a twist or a bend thereof. The mechanical stress is then applied to the printed wiring substrate. Stress is thereby generated in solder junction portions of the corner portions of the CSP. In order to increase strength of the corner portions to prevent the stress from being generated therein, reinforcing pads (PAD) consisting of copper chip patterns are conventionally formed on predetermined portions of the printed wiring substrate with the reinforcing pads (PAD) facing the corner portions of the CSP.
As will later be described more in detail, a first conventional semiconductor device using the CSP comprises a CSP body portion and a plurality of solder balls formed on a lower surface of the CSP body portion at predetermined pitches. The CSP is mounted on a printed wiring substrate by connecting the solder balls to conductor patterns on the printed wiring substrate by the use of soldering.
The reinforcing pads are formed, for example, on the four corners of the lower surface of the CSP body portion. A plurality of the solder balls, which are formed on the lower surface of the CSP body portion at predetermined pitches, are also mounted on each of the reinforcing pads, respectively. The reinforcing pads improve connection strength of the solder balls by increase of soldered areas. As a result, the reinforcing pads can release stress applied on soldered portions around the reinforcing pads, respectively.
However, in the first conventional semiconductor device mentioned above, when mechanical stress is applied near the reinforcing pads, the mechanical stress (tension stress) is applied on the reinforcing pads. In particular, when the mechanical stress is concentrated on an edge portion of the reinforcing pads, the solder balls are inevitably removed therein (junction plane by soldering is removed).
An example of a second conventional semiconductor device is exemplified in unexamined Japanese patent publication No.2000-200854. In the example, the second conventional semiconductor device has some features in order to improve reliability of soldering connection between CSP and a printed wiring substrate and to enable the second conventional semiconductor device to be contained in a conveyer tray without damaging solder bumps. For example, bump-less pads having no solder bump are provided on the corners of the CSP in the example. Further, solder balls for reinforcing are provided in the positions facing the bump-less pads on the printed wiring substrate. Herein, each of the solder balls has a diameter larger than that of a solder bump provided in the CSP side. In mounting processes, when the CSP and the printed wiring substrate are junctioned and then subjected to a reflow process, an adhered area of each solder ball for reinforcing having the larger diameter is enlarged, compared with the other solder balls each having a smaller diameter. Accordingly, mounting strength is thereby increased.
Further, an example of a third conventional semiconductor device is also exemplified in unexamined Japanese patent publication Hei1-288978, namely No.288978/1999. In the example, in order to improve mounting strength in mounting the third conventional semiconductor device on a mother substrate, recess portions are provided on the four corners of an interposer substrate. In addition, electrodes are formed in the four corners of the interposer substrate, respectively. The electrodes are then connected to be a pad formed on the mother substrate by soldering.
However, in the second conventional semiconductor device exemplified in the unexamined Japanese patent publication No.2000-200854, it is necessary to provide solder bumps both on the CSP and on the printed wiring substrate. Manufacturing processes thereby become complicated.
On the other hand, in the third conventional semiconductor device exemplified in the unexamined Japanese patent publication Hei1-288978, namely No.288978/1999, it is necessary to form the electrodes, respectively on the four corners of the interposer substrate. In addition, it is also necessary to conduct the soldering process between the electrodes and the pads on the mother substrate.
It is therefore an object of the present invention to provide a semiconductor device which is capable of reducing remove of solder balls in a reinforcing pad by preventing mechanical stress from being concentrated on a part of the reinforcing pad.
Other objects of the present invention will become clear as the description proceeds.
According to an aspect of the present invention, there is provided a semiconductor device which is mounted on a principle surface of a substrate to be connected with a circuit formed on the principle surface, the semiconductor device comprising: a body portion having a mounting surface; a plurality of solder balls which are formed on the mounting surface and which connect the semiconductor device to the principle surface of the substrate; a reinforcing pad provided on the mounting surface and having portions on which the a plurality of solder balls are positioned; and circumferential shapes of the portions on which the a plurality of solder balls are positioned being hemmed to have roundness in line with outer diameters of the a plurality of solder balls, respectively.
The circumferential shapes of the portions on which the a plurality of solder balls are positioned may be hemmed at least halfway round each of the a plurality of solder balls.
The reinforcing pad may have a cross shape, an X-shape, an L-shape, or a V-shape.
The body portion may be CSP (Chip Size Package).